The A&R system has the function of moving the pipeline relative to the tower such that the pipeline is lowered from the tower to the seabed (abandoning the pipeline) or raised from the seabed to the tower (recovering the pipeline). Such an abandonment and recovery procedure is often named an A&R procedure.
An A&R operation can occur for various reasons, for instance to place the pipeline in its final position on the seabed after completion, or to lay down a pipeline in order to pick it up and complete it at a later time or with a different vessel. Further, the laying of the pipeline is not possible during bad weather conditions. During bad weather it is not always possible to keep the pipeline suspended from the vessel. Therefore, when bad weather is forecasted, the pipelaying operation has to be stopped and the pipeline has to be abandoned to the seabed. This means that the pipeline is abandoned from the vessel by the A&R system such that said pipeline is positioned on the seabed. The pipeline may remain connected to the vessel via for example a cable. When the weather conditions are improved, the A&R system is used to raise the pipeline from the seabed to resume the pipelaying. Such an A&R procedure is very time consuming. Due to this, much costly operational time in which pipeline can be laid is lost.
In a known vessel, the A&R line is supported by the tower near the upper end thereof. In said known vessel, the A&R line is reeved from an A&R winch located on the vessel deck, through the tower and into the firing line by means of sheaves. One sheave assembly is located near the upper end of the tower and the A&R line is supported by the tower via said sheave assembly. The sheave assembly is located near the upper end of the tower, so that space is provided for the addition of one or more A&R tools to the pipeline. Said A&R tools are used for facilitating the A&R operation and may comprise an A&R head which may be welded on the pipeline and/or associated rigging for connecting or disconnecting the A&R system to or from the pipeline.
During an A&R procedure, the weight of the pipeline is carried by the A&R system. The loads which the A&R systems have to bear are nowadays still increasing since pipelines are laid in increasing water depths (more than 3000 m). The high forces to which the A&R system in use is subjected are in the known vessel transferred to the tower via the sheave assembly. To withstand said forces, a tower with a heavy construction is required. Such a tower is expensive to build and difficult to handle.